Study of milk somatic cell count pattern associations with some of environmental factors in Holstein dairy cattle of Iran

The objective of the present study was to estimate phenotypic trend and investigate environmental factors affecting milk somatic cell counts in Holstein dairy cattle of Iran. This research was conducted in a large scale using dataset collected by Animal Breeding Center of Iran. The dataset consisted of 4,279,953 test day records of somatic cell counts (SCC) related on 1,873,234 cows from lactations one to four gathered from 2002 to 2013 on 1281 dairy farms in Iran. All statistical analyses were performed using different procedures (Proc GLM, Proc Reg and Proc Univariate) of SAS software by including somatic cell score (SCS) as the dependent variable. All factors in the model including herd, parity and year of calving, month of recording, milk yield, days in milk (DIM) and age at first calving (AFC) had  significant effects on SCS (P<0.01). Results of this study indicated that by increasing parity number, SCS increased. The lowest and highest levels of SCC were observed in months of Bahman (21 January-19 February) and Mehr (23 September-24 October), respectively. There was a linear relationship between AFC and SCS, so that by increasing AFC, the SCS increased. As daily milk yield levels increased, the SCC has reduced. The results obtained in this research revealed that pattern of phenotypic trend for SCS through lactation were inverse of milk lactation curve i.e. as DIM increased, SCC reduced. The pattern of phenotypic trend for SCS through investigated years was descending. This trend can partially be due to improvement in management factors and mastitis control programs that are widely being used across the country dairy farms.